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South Atlantic Anomaly

The South Atlantic Anomaly (SAA, the large red area in the image) is a dip
in the Earth's magnetic field which allows cosmic rays, and charged particles
to reach lower into the atmosphere. This interferes with communication with
satellites, aircraft, and the Space Shuttle. While there are theories as to
why this occurs, the geologic origin is not yet known.

The enhanced particle flux in the SAA also strongly affects X-ray detectors,
which are in essence particle detectors. The ROSAT PSPC had to be turned
off during passage through the SAA otherwise it would have suffered severe
damage. While the ROSAT HRI could be left on during the passage, it could
collect no useful data. The light blue and green bands at the top and
bottom of the image are due to an enhanced particle flux above Earth's
auroral zones (particle belts).

The data were collected by the South Atlantic Anomaly Detector (SAAD)
aboard ROSAT. It consists of 10 cm² of Germanium and served as a particle
background monitor. The SAAD was used to "safe" the PSPC if rates became
too high.